Hydronic space heating systems comprise a boiler that delivers hot water throughout an area to be heated. Current applications include wall mounted radiators or tubes embedded in the floors of residences or industrial sites for distributing the hot water throughout the home or building. A substantial portion of boilers for hydronic heating systems are oil or gas fired, using an external burner that generates hot gases and delivers the hot gases to a boiler to elevate the temperature of the water. Such oil and gas burners have been developed to a high level of sophistication to provide safe, reliable, and efficient burning of the fuel, with extremely low levels of emissions.
Combustion of fossil fuels such as coal, oil and natural gas generates substantial quantities of carbon dioxide. This contributes to the environmental problem of "global warming" that results from the excessive build-up of these "greenhouse gases." Wood is commonly used as a residential heating fuel in areas where wood is readily available and economical. Use of wood has some obvious drawbacks due to the need for frequent manual loading of the fuel, storage of the fuel, variability in the moisture of the fuel and the need to remove ashes resulting from the combustion of the fuel.
Wood pellets, a dry, processed, wood fuel made from waste sawdust, chips, etc., can be an economical and environmentally safe fuel substitute for fossil fuels for the purpose of residential and industrial space heating. In contrast to fossil fuels, which contribute to the buildup of greenhouse gases, wood is considered a "renewable fuel" that contributes no net buildup of atmospheric carbon dioxide.
The use of these wood pellets in heating stoves is widespread. One example of such type of stove is described in U.S. Pat. No. 5,488,943, by the applicant of the subject application. In these types of wood stoves, which are generally free standing or fireplace inserts, the wood pellets are combusted to provide hot gas which is passed through a heat exchanger to transfer thermal energy to room air that occupies the opposite side of the heat exchanger. Users of these types of pellet fuel heating stoves find them desirable because of the aesthetic advantages of a natural flame that is readily visible through the door of the stove.
The attractiveness of wood as an alternative to fossil fuel has led to the development of several hydronic heating systems that employ boilers that are fueled by wood pellets. These systems provide an environmentally friendly alternative to existing fossil fueled boilers. In those instances where the boilers themselves do not need to be replaced, the user must decide whether his or her desire for a burner fueled by a renewable fuel is outweighed by the cost of replacing an otherwise satisfactory fossil fuel burner. Given the large number of fossil fuel boilers in use, it would be desirable if a hot gas generator fueled by wood pellets was available which could be used to replace the fossil fuel burner of existing boilers. This would avoid the cost of replacing the entire boiler unit and would result in less waste of valuable resources. There have been attempts to design pellet fuel burners that would replace fossil fuel burners in existing boilers; however, such designs generally attempt to place the actual burner grate within the boiler. This approach suffers from the drawback that a specific burner design is required for each unique boiler design.
In hydronic heating systems, the heat transfer efficiency from the hot gases produced by the combustion of fuel to the boiler decreases proportionately to the quantity of air used in the combustion process. Oil burners typically utilize 15%-30% excess air above theoretical "stoichiometric" requirements and generate emissions of carbon monoxide and nitrogen dioxide on the order of 50 parts per million. First generation designs of boilers using wood pellets as a fuel use 200% excess air or higher and can emit carbon monoxide levels in excess of 500 parts per million.
With the concern for "global warming" and the push to implement alternative fuel sources which are renewable and environmentally friendly, prior designs of burners using wood pellets as a source of hot gases for hydronic heating systems have met limited acceptance because of the high levels of excess air required, lower efficiencies, high levels of carbon monoxide they emit, and the need for numerous designs to correspond with the multitude of boiler designs, even though they employ a renewable fuel source.